I
R. S. MONTGOMERY, D. 1. DECKER, and J. C. MACKEY The Dow Chemical
Co., Midland, Mich.
Chemicals from Coal
Ethylene and Aromatics
by Carbonization of Lignite
This process will be commercially important if the cost of crude oil increases enough to offset the higher initial capital cost E T H Y L E N E and aromatics are now made from crude oil, but because of dwindling supplies and higher prices for crude oil, another method is desirable. A new process involves low temperature carbonization of lignite, followed by high temperature cracking of the tar vapors.
Equipment The evolved volatile matter passed from a carbonizer through the cracker into the collection train (7), which consisted of a water-jacketed receiver, spiral condenser, electrostatic precipitator, and another spiral condenser. From there the gas passed through a cotton trap to remove uncondensed fog and mist, through a dry ice-acetone trap to condense low boiling constituents, and finally through a wet-test gas meter. The carbonizer consisted of a 2-inch 466 stainless steel pipe 24 inches long. Extending from the bottom of the retort to within 6 inches of the top was a '/*-inch stainless steel pipe, exit for all the volatile matter. A 1-inch stainless steel plug fitted with a thermowell, TR1, was used a t the top of the retort for introduction of the lignite charge and removal of the char. A second thermowell, TR 2, was provided through the bottom of the carbonizer, the innermost point being welded to the '/k-inch pipe. The carbonizer was heated by electric furnaces controlled with Variacs. T h e temperatures were measured with thermocouples, and recorded on electronic recorders. Two cracking reactors, 18 and 36 inches long, were fabricated of Vycor tubing of 31-mm. outside diameter. A concentric 9-mm. Vycor tube, through the entire length of the cracking tube, allowed a thermocouple to be placed inside the tube and the cracking temperature, TC, to be recorded continuously on an electronic recorder for any point in the tube. The temperature was measured a t a point "3 of the distance from the entrance of the cracker to the exit. The cracker was heated with resistance tape wound directly on the tubing and the electric current controlled by a Variac.
Texas lignite is superior to North Dakota lignite in this process; it produces almost twice as much ethylene and about ten times as much benzene. The carbonization products must be cracked without being allowed to condense, to avoid production of char and residue in the cracker. Best results were obtained using carbonization temperatures up
to 550' C., a cracking temperature of 800" C., and a cracker length of 36 inches, which resulted in an average retention time of about 1.5 seconds. These conditions would probably be altered somewhat in a large commercial unit. However, the performance of a large unit can be estimated on the basis of the experimental results.
T o condense the mist in the gas from the cracker, a small electrostatic precipitator was used. I t consisted of four parallel 12-inch lengths of stainless steel tubing inch in outside diameter, in water-jacketed borosilicate glass housings. Suspended coaxially in the tubes were platinum wires, connected to a negative direct current potential of 3000 volts. The stainless steel tubes were connected to ground.
31.1%. The dried lignite was thcn crushed and separated into fractions based on particle size. For the carbonizations, the fraction was used that passed 3l/2-mesh screen and rcrnained on 10-mesh screen.
lignite Used Texas lignite was obtained from Bastrop County and the North Dakota lignite from the Truax-Traer Coal Co., Minot, N. D. The samples were obtained in sealed drums in a n "as mined" state and were of 3-inch particle size. After drying overnight a t 100' C. in a vacuum oven a t a pressure of 3 to 5 mm., Texas lignite lost 29.3% of its weight and North Dakota lignite lost
Procedure Processed lignite (500 grams) \vds charged into the carbonizer and thc cracker temperature brought up to the predetermined level. Then, the temperature of the carbonizer was slo~vlyand uniformly raised from room temperature to a maximum of 800' C. About 70 minutes was required for TR2 to increase from 300' to 800' C. Electric current to the carbonizer furnaces was controlled manually, so that 7R1 equaled TRz. The carbonization temperature was stabilized a t 800' C. and maintained there for another hour. After this time no further gas evolution was observed.
The equipment consisted of an electrically heated carbonizer immediately followed by a ternperature controlled cracker and a collection train
VOL. 51, NO. 10
OCTOBER 1959
1293
the wet-test gas meter; samples i t taken every 12 liters and analyzed l)y mass spectrornetr)-.
The Lignites Were Partially Dried before They Were Carbonized
Texas Proximate Moisture Volatile matter Fixed carbon Ash Ultimate Carbon Hydrogen Oxygen Nitrogen Sulfur Ash
Dakota -_ North *I'.
lbn.
.bcharged
Ihigh cracking remperatures and longer retention rime, conditions {vhich would be expected to favor the conversion of saturated to unsaturated hydrocarbons. Production \vas almost identical from both lignites. Carbon Monoxide. .As carbonization temperature is increased, the proporrion of carbon monoxide in the evolved gas shows a minimum of 540' and a well defined maximum a t 670 O C. a t all cracking temperatures and retention times. Production is almost completely insensitive to cracking condirions. Texas and North Dakota lignites behave very similarly, although slightly more carbon monoxide is obtained from Korth Dakota lignite a t carbonizer temperatures below 600" C.
The
550"
0.2 0.9 1.2 0.6 0.3 0.6 0.6 0.5 0.8 0.9 _8_ . 9_ 15.5
0.6 0.7 0.3 0.1 0.3 0.6 1.4 0.2 0.6 0.3 __ 6.3 11.4
0.5 1.0 1.6 1.0
1 .o
0.2
0.5
Carbon Dioxide. Less cart~cindioxide is evolvrd as the carbonizer temperature is raised, until essentially none is evol\,ed above 7 3 0 " C . T h e proportion of carbon dioxide in the evolved gases beloiv a carbonizer temperature of .550 C;. is loi\-er a t the higher cracking remperatures, althougli this is due entirely to the greater amounr o l total gas produced undrr these conditions. Xfore carbon dioxide is obtained from S o r t h Dakota lignite tlian from Texas lignite. Hydrogen Sulfide. Onl>-rather sinall amounts of hydrogen sulfide are obtained, almost all below a carbonization temperature of 600 ' C. T h e proportion in the evolved gas is affkctrd b y tlie cracking conditions in Texas lisnite, but not in North Dakota lignitc. .Approximately txvice as much hydroSen sulfide is also obtained from the Texas lignite. Steam-Distillable Oil. Lcss is obtained a t the higher cracking temperatures. The retention time had litrlc effect on the amount produced from North Dakota lignite but significantly more \vas produced from Texas lignite at the longer retention rime. . i t the loneer retention time, approximately t\vice as
Tevns
.j511° C .
Proximate Moisture Volatile matter Fixed carbon Ash Ultimate Carbon Hydrogen Oxygen Nitrogen Sulfur Ash Calorific value, B.t.u./lb.
SIKlO
c
__S l ~ l t l l1)- IhOt'l - .-.-
550' c'
suoc (
0.0 11.4 68.6 20.0
0.0 4.1 72.2 23.7
0.0 14.8 72.8 12.4
0.0 4.0 82.2 13.8
72.3 2.7 2.8 1.3 20.0
73.3 0.9 0.3 0.9 0.9 23.7
78.0 2.9 4.9 1.3 0.5 12.4
83.4 1.3 0.1 0.9 0.5 13.8
12,035
11,443
12,571
12,608
0.9
0.2 0.2 0.5 45.6 52.3
-
C. Char Had Enough Volatile Matter to Be Burned without Difficulty
-
1296
0.2 0.7 1.4 0.9 0.7 0.2 0.2 0.2 0.2 0.5 ___ 17.5 22.7
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.4 1.1 2.3
1.5 0.8 0.4 0.8 0.4 0.8 1.1
__ 28.6 38.2
1.0 1.5 2.0 1.5 1.0 0.5
0.5 0.5 0.5 0.5 39.6 49.1
0.2 1.2 1.4 0.4 0.4 0.9 1.1 0.5 1.2 1.1 9.4 17.8
6.0 4.9 2.3 0.4 1.1 1.9 3.0 1.1
1.9 0.7 1 4.4. . 37.7
Char from Texas and North Dakota Lignites
